Sialyltransferases are a group of glycosyltransferases that transfer sialic acid from an activated sugar nucleotide to acceptor oligosaccharides found on glycoproteins, glycolipids or polysaccharides. Sialylated oligosaccharides play important roles in cell-cell recognition, cell differentiation and various receptor-ligand interactions in mammalian systems. The large number of sialylated oligosaccharide structures has lead to the characterization of many different sialyltransferases involved in the synthesis of these structures. Based on the linkage and acceptor specificity of the sialyltransferases studied so far, it has been determined that at least 13 distinct sialyltransferase genes are present in mammalian systems (Tsuji, S. et al. (1996) Glycobiology 6:v-vii).
Sialylated glycoconjugates are also found in bacteria (Preston, A. et al. (1996) Crit. Rev. Microbiol. 22:139-180; Reuter, G. et al. (1996) Biol. Chem. Hoppe-Seyler 377:325-342) are thought to mimic oligosaccharides found in mammalian glycolipids to evade the host immune response (Moran, A. P. et al. (1996) FEMS Immunol. Med. Microbiol. 16:105-115). The importance of sialylated lipooligosaccahride (LOS) in the pathogenesis of Neisseria gonorrhoeae has been established (Smith et al., (1992) FEMS Microbiol Lett. 100:287-292) while for N. meningitidis both the polysialic acid capsule and the sialylated LOS were found to be important for pathogenicity (Vogel, U. et al. (1996) Med. Microbiol. Immunol. 186:81-87).
Despite their importance as proven or potential virulence factors, few bacterial sialyltransferases have been cloned (Weisgerber, C. et al. (1991) Glycobiol. 1:357-365; Frosch, M. et al. (1991) Mol. Microbiol. 5:1251-1263; Gilbert, M. et al. (1996) J. Biol. Chem. 271:28271-28276) or purified (Yamamoto, T. et al. (1996) J. Biochem. 120:104-110). The .alpha.-2,8-sialyltransferases involved in the synthesis of the polysialic acid capsules have been cloned and expressed from both Escherichia coli (Weisgerber, C. et al. (1991) Glycobiol. 1:357-365) and N. meningitidis (Frosch, M. et al. (1991) Mol. Microbiol. 5:1251-1263). Glycosyltransferases from N. gonorrhoeae involved in the synthesis of lipooligosaccharide (LOS) have been cloned (U.S. Pat. No. 5,545,553).
Because of biological activity of their products, mammalian sialyltransferases generally act in specific tissues, cell compartments and/or developmental stages to create precise sialyloglycans. Bacterial sialyltransferases are not subject to the same constraints and can use a wider range of acceptors than that of the mammalian sialyltransferases. For instance, the .alpha.-2,6-sialyltransferase from Photobacterium damsela has been shown to transfer sialic acid to terminal galactose residues which are fucosylated or sialylated at the 2 or 3 position, respectively (Kajihara, Y. et al. (1996) J. Org. Chem. 61:8632-8635). Such an acceptor specificity has not been reported so far for mammalian sialyltransferases.
Bacterial glycosyltransferases are useful in a number of applications, such as the synthesis of desired oligosaccharides with biological activity. Identification and characterization of new bacterial glycosyltransferases is thus useful in the development of these technologies. The present invention provides these and other advantages.